Retarders are used in drive-trains of commercial vehicles as wear-free braking devices and are then predominantly in the form of hydrodynamic retarders. In a hydrodynamic retarder the flow energy of fluid is used to produce a braking torque and for that purpose the fluid is introduced into a working space between a stator and a rotor. In the working space, which is usually of torus-shaped design, the rotating rotor entrains the fluid by means of vanes and directs it onto vanes of the stator, against which the liquid impinges and is then propelled back onto the vanes of the rotor. The returning liquid has a decelerating effect on the rotor, such that the size of a braking torque is controlled by the quantity of fluid introduced into the working space.
However, even when the working space is not full the rotating rotor produces a certain amount of braking torque which is caused by the pumped circulation of air and fluid residues between the rotor and the stator. To avoid these losses, also known as air losses, apart from during braking operation hydrodynamic retarders are partially decoupled from the remainder of the drive-train so that no rotation of the rotor takes place and the aforesaid pumped circulation is avoided. In some cases, for this the retarder is designed to be connected and disconnected within the motor vehicle transmission of the commercial vehicle concerned.
DE 10 2009 001 147 A1 describes a drive-train of a commercial vehicle, in which a hydrodynamic retarder and an electrodynamic retarder can be connected and disconnected within a motor vehicle transmission. For that purpose a spur gearwheel is mounted to rotate on a transmission output shaft, which can be fixed to the transmission output shaft by means of a shifting element and which meshes with each of two fixed wheels. Of the two fixed wheels one is fitted on a drive input shaft of the hydrodynamic retarder and one on a drive input shaft of the electrodynamic retarder. Together with the two fixed wheels the spur gear mounted to rotate on the transmission output shaft forms two gear steps by virtue of which, when the spur gearwheel is fixed to the transmission output shaft, rotation of the transmission output shaft is transmitted to the respective drive input shaft of the hydrodynamic retarder and the electrodynamic retarder. In turn, the transmission output shaft can be coupled by way of various gear steps of the motor vehicle transmission to a transmission input shaft, which can be connected on the drive input side by way of a friction clutch to the driveshaft of an internal combustion engine.